The present invention relates to optoelectronic devices for detecting or emitting light, such as an avalanche photodiode, other photodiodes which are used as photodetectors in fiber-optical communication, and a laser diode.
In order to lower the noise level of an avalanche photodiode type, and to increase the operation speed of the avalanche photodiode, an avalanche photodiode made of compound semiconductor materials has been developed. In this avalanche photodiode, incident light is absorbed by a semiconductor region having a relatively small band gap, and a photocurrent thus obtained is amplified on the basis of the avalanche multiplication phenomenon in another semiconductor region having a band gap larger than the band gap of the light absorbing region. Semiconductor materials such as an InP compound and a GaSb compound are used for making the above avalanche photodiode. Further, in order to improve the performance of the above avalanche photodiode, a guard ring region for preventing the avalanche breakdown on the periphery of a pn junction is formed in the avalanche photodiode, and means for suppressing the carrier accumulcation effect based upon the difference in energy gap between the light absorbing region and the avalanche multiplication region to increase the operation speed of the avalanche photodiode is provided therein, that is, the region 4 of FIG. 6 having a band gap which is intermediate between the band gap of the light absorbing region and that of the avalanche multiplication region, is interposed between these regions.
However, in a compound semiconductor which has a small band gap and a small effective electron mass, there arises the following problem. That is, when a strong electric field is applied in the compound semiconductor, breakdown due to the tunneling effect is apt to occur before the avalanche breakdown is generated. Accordingly, when a reverse bias voltage applied across an avalanche photodiode with the SAM structure (namely, the separated absorption and multiplication structure) is increased so that a strong electric field extending from a pn junction which is formed in a semiconductor region having a large band gap (namely, the region 5 of FIG. 6), is applied to a middle region (namely, the region 4 of FIG. 6) or light absorbing region (namely, the region 3 of FIG. 6) having a small band gap, a dark current is increased by the field emission due to the tunnel effect. In order to prevent such an increase in dark current, the impurity concentration of that portion of the semiconductor region having the large band gap (namely, the region 5 of FIG. 6) which exists at a portion between the pn junction and the middle region or light absorbing region, has been made high, or the thickness of the above portion has been made large. However, when the impurity concentration of the above portion is made high, a maximum electric field intensity at the pn junction becomes large. Then the ratio k of the ionization coefficient of hole .beta. to that of electron .alpha., k.ident..alpha./.beta., is decreased. Here, the ionization coefficient means the number of electron-hole pairs when one hole (or electron) transits a unit length. As a result, the avalanche magnification noise is increased (the noise of the avalanche photodiode is low when the ratio k is high). When the thickness of the above portion is made large, the operating voltage applied across the avalanche photodiode becomes high. Such a high operating voltage is undesirable from a practical point of view.